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Once searchers hunting for the crashed Malaysian jet decide to shift from listening for the acoustic signals from the black boxes from the floor of the Indian Ocean to poring over its treacherous terrain, they will have to draw from a whole new set of tools, experts say.

Australian defence vessel Ocean Shield has been using a US Navy towed pinger locater to listen out for possible signals from MH370, which were detected twice last weekend and twice on April 8.

But no new signals have been confirmed since then, prompting experts to conclude that the batteries of the Boeing 777-200's black boxes may have died because the battery life usually lasts for only a month - and that window has passed.

The Malaysia Airlines Flight 370 was flying from Kuala Lumpur to Beijing on March 8 when it lost contact with air traffic controllers and mysteriously vanished from radars.

Officials believe the plane flew off course for an unknown reason and went down off the west coast of Australia in the southern Indian Ocean.

Finding the black boxes is crucial to know what happened before the Beijing-bound Malaysia Airlines jet with 239 people, including five Indians, mysteriously vanished after taking off from Kuala Lumpur.

Now, searchers may have to deploy the American Bluefin- 21, a probe equipped with side-scan sonar -- an acoustic technology that creates pictures from the reflections of sound rather than light.

"That is a piece of equipment that does assist in locating where the wreckage may be," said Sylvia Earle, an oceanographer from National Geographic who was chief scientist for the National Oceanic and Atmospheric Administration.

Though the discovery of four pings, believed to be from the Flight MH370's black boxes -- its flight data recorder and its cockpit voice recorder -- have helped investigators narrow the search area, they would still face a formidable task to track the plane at depths of over 4,500 meters.

"It's a lot of terrain to cover," given that the Bluefin -21 moves at the pace of a leisurely stroll, Earle said.

Though the underwater drone moves slowly, it creates good images -- so good that they are "almost a picture of what's there ... but it's imaged with sound instead of with a camera."

Once the debris field is found, then other equipment -- such as remotely operated vehicles -- would be brought in to recover the black boxes, Earle was quoted as saying by CNN.

Remotely Operated Vehicles working at depths of three miles would require power conveyed down a cable from a ship above, she said. "There are not many pieces of equipment in the world able to do this."